1. Field of the Invention
The present invention relates to a recycling method and a recycling apparatus of slurry which is used in the polishing process of a manufacturing process of semiconductor wafer and, more specifically, a recycling method and a recycling apparatus, which permits the reuse of a large amount of used-slurry in a short time.
2. Description of the Related Art
In the manufacturing process of semiconductor wafer, in general, CMP (Chemical Mechanical Polishing) is used for wafer final polishing. In the CMP, final polishing is performed to make a wafer surface into a mirror surface by pressing an abrasive pad onto the wafer held on a pedestal, supplying slurry as abrasive to between the wafer and the abrasive pad, and rotating the pedestal and the abrasive pad independently.
In the polishing process, the slurry containing colloidal particles (silica, etc.) to which water-soluble polymers such as cellulose are added is generally used for polishing, in order to make the wafer surface into a mirror surface with high flatness.
Since the slurry containing water-soluble polymers is subjected to agitation or pressure in the course of polishing, the water-soluble polymers or colloidal particles clump to generate aggregates. Since the slurry used in the polishing process contains aggregates and there is fear that foreign substances are brought into the slurry, resulting in the contamination of semiconductor wafers, it was not possible to recycle the used-slurry and it is inevitably disposed of as wastes.
In recent years, it has been attempted to recycle used-slurry from the viewpoint of reducing the preparation cost and disposal cost of slurry and protecting the environment. For example, Japanese Patent Application Publication No. 2002-170793 proposes a recycling method of used-slurry, comprising: removing supersize aggregates contained in the used-slurry with a filter, and centrifugally separating and condensing the used-slurry into liquid concentrate of slurry.
Japanese Patent Application Publication No. 2002-331456 proposes a recycling method of used-slurry, comprising: condensing the used-slurry into liquid concentrate of slurry by a membrane separation means, diluting and washing the liquid concentrate of slurry with pure water followed by re-concentrating, and adjusting the pH by adding an alkaline chemical agent.
Further, Japanese Patent Application Publication No. 2004-63858 proposes a method for recycling used-slurry without using a filter by emitting an ultrasonic beam to the used slurry to break up aggregates in the slurry, and then removing the remaining aggregates by sedimentation while adjusting the temperature of the used-slurry.
Japanese Patent Application Publication No. 2004-75859 proposes a recycling method of slurry for use in wafer polishing, comprising: removing metal ions existing in used-slurry, with the aim of preventing contamination of a semiconductor wafer surface, by filtering the used-slurry through chelate forming fibers.
However, any of the methods proposed in these literatures could not entirely solve the following problems, and were inadequate for the reuse of used-slurry.
(1) The composition of slurry changes with the lapse of time since the slurry reacts readily with air (particularly, carbon dioxide).
(2) In case of the filtration by a filter, frequent replacement of the filter is needed since clogging of the filter with aggregates occurs in a short time.
(3) If metal-ion-containing foreign substances are mixed to the slurry in the polishing process, the resulting semiconductor wafer is contaminated therewith, causing product defects.
Therefore, Japanese Patent Application Publication No. 2009-54629 proposes a recycling method of slurry for use in wafer polishing, comprising: adding a dispersant for inhibiting the aggregation of slurry to the recovered used-slurry; emitting an ultrasonic beam toward the resulting slurry to break up aggregates in the slurry; further adding a chelating agent to the used-slurry to remove metal ions; and thereafter removing foreign substances in the slurry with a filter. Thus, it is perceived that according to the method for recycling slurry for use in wafer polishing of this literature, all the above-mentioned problems (1) to (3) can be solved.